1. Field of the Invention
The present invention relates to an optical fiber connector for positioning and connecting optical fibers. More specifically, the invention relates to an optical fiber connector which can connect a plurality of opposed optical fibers simultaneously with a low connection loss.
2. Description of the Prior Art
In connecting optical fibers with an optical fiber connector, it is required (1) to reduce a connection loss at the point of connection (i.e. to achieve low connection loss) and (2) to reduce backward reflection at the point of connection (i.e. to achieve low reflection).
With a single-fiber connector, therefore, physical contact (PC) coupling, i.e., joining of the end faces of optical fibers in direct contact, has been put to practical use. For example, a polishing disk for an optical fiber connector as shown in Japanese Patent Application Laid-Open No. 137107/1986 has been developed.
With a multifiber connector, on the other hand, as the number of optical fibers increases, it becomes more difficult to achieve PC coupling with all fibers. Thus, (i) an attempt to achieve low reflection has been made by coupling optical connectors while interposing an index matching material, which has practically the same refractive index as that of the core of an optical fiber, between the end faces of optical fibers; or connecting optical fibers with their end faces being polished in an inclined shape. (ii) An attempt to achieve PC coupling which involves a low connection loss has been to protrude optical fibers slightly as described later.
However, the application of the index matching material for such connection as described in (i) above is a disadvantage to operating efficiency and handling. In the case of (ii), it is difficult to attain a complete PC coupling for all optical fibers in any combinations of connectors, if the connectors are of the same make.
An optical fiber connector as shown in FIGS. 1 and 2 has been provided under these circumstances (a first embodiment in Japanese Patent Application Laid-Open No. 336509/1992, FIGS. 1 and 3). In the drawings, the numerals 1, 1' each denote a plastic molded multifiber ferrule, 2 a pair of guide pin insertion holes, 3 an end face of the ferrule, 4 an optical fiber tape, and 5 a plurality of optical fibers housed in the optical fiber tape 4. In FIG. 1, the plural optical fibers 5 in the optical fiber tape 4 are arranged and fixed in the ferrule 1 so as to be located between the pair of guide pin insertion holes 2. An end face 3 of the ferrule 1 is inclined at an angle .theta. to a plane perpendicular to the axis of the optical fiber 5, the angle being larger than the total reflection critical angle of propagated light in the optical fiber 5. A end face 5a of the optical fiber 5 slightly projects from the end face 3 parallel thereto.
According to this conventional optical fiber connector, the end faces of optical fibers are in intimate contact with each other. The light which is reflected from the inclined surface of the angle .theta. and which is attributable to a processing distortion layer having a high refractive index due to the polishing of the end faces of the fibers is considered not to propagate to the light source side, since this reflected light has an angle larger than the total reflection critical angle relative to the axes of the optical fibers. Thus, the conventional optical fiber connector has been regarded as realizing low reflection and low connection loss without using an index matching material.
The above conventional multifiber connector protrudes a plurality of optical fibers slightly from the end face of the ferrule, and brings the cores of the optical fibers into contact (PC coupling). This type of connector presents the problem that the larger the number of optical fibers, the more greatly the connection loss varies.
This is because the protruding lengths of the plural optical fibers are not necessarily identical, but are slightly different. The difference in the protruding lengths of the plural optical fibers is apt to cause a failure in providing a complete PC coupling of the optical fibers when they are connected. Such difference may also render the PC coupling incomplete when the connector are attached to or detached from each other, or when an external force such as bending is exerted.
When the PC coupling is not complete, the Fresnel reflection generates a connection loss of 0.3 dB in connecting the optical fibers. Multiple reflection, if any, between the optical fibers causes a connection loss of 0.6 dB at the greatest. Thus, whether the PC coupling of optical fibers is complete or not leads to variations in the connection loss.
The causes of such variations in connection loss are further explained in detail as follows:
(1) In the production of an optical fiber connector, the end faces of the optical fibers and the ferrule are polished for finishing. As a result of polishing, the optical fibers slightly protrude from the end face of the ferrule mainly because of the difference in hardness between the ferrule and the optical fiber. The protruding lengths of optical fibers become different for the reasons stated below.
(i) The polishing disk, as described earlier, is used to polish the ferrule for contact (PC coupling) between the cores of optical fibers so that a plurality of optical fibers are slightly protruded from the end face of the ferrule. For this purpose, the polishing disk is constituted to be elastic and relatively small in thickness. Thus, the polishing disk undergoes elastic deformation during polishing of the ferrule end face. As a result, the end face of the ferrule has a mountain-shaped shear droop after polishing, as shown in FIG. 3.
(ii) Variations in the polishing speed cause different protruding lengths of optical fibers as illustrated in FIG. 4.
(2) If the protruding lengths of optical fibers from the end face of the ferrule are different for the above causes, the PC coupling of optical fibers becomes difficult. The failure to achieve a complete PC coupling of the optical fibers results in the creation of an air layer between the optical fibers to be connected, which causes connection losses due to the Fresnel reflection as described earlier.